In a typical fuel control system of a gas turbine powerplant for an aircraft, a fuel control system modulates the fuel flow to the powerplant in response to the operator's demands. The operator's thrust demands are input to an electronic engine control (EEC) which monitors various engine parameters to determine the amount of fuel flow required to meet the operator'thrust demand. The main objective of the fuel control system is to be responsive to the operator's thrust demand while at the same time prevent engine stall, engine surge, or inefficient operation of the powerplant.
The EEC receives the operator's thrust demand and calculates a desired engine parameter which correlates to the thrust of the powerplant. Engine pressure ratio, referred to as EPR, is one such engine parameter although others, such as shaft rotational speed, may be used. EPR is the ratio of engine output pressure to engine inlet pressure. An EPR reference signal, which corresponds to the EPR required to meet the operator's thrust demand, is produced by the EEC. EPR reference is compared to a sensed or actual EPR taken from pressure sensors mounted at the inlet and outlet of the powerplant. The comparison generates an EPR error signal. EPR error is manipulated by the control system to generate a metering valve position reference referred to as a requested degrees resolver signal (DR). DR is compared to a sensed or actual DR to generate a degrees resolver error (DRE) signal. DRE corresponds to the amount of displacement of the metering valve required to meet the operator'demand. A torque motor mechanically positions the metering valve in response to a command current signal. The command current signal is generated by the fuel control system and correlates to the DRE.
In essence, the fuel control system is comprised of two feedback loops. The major loop includes a comparison of the sensed EPR to the EPR reference to produce the EPR error. The minor loop includes the comparison of the sensed metering valve position to the DR to produce the DRE. A failure in the minor loop results in the fuel flow being switched to a fail-safe fuel flow. For safety reasons, the fail-safe position for aircraft powerplants is a minimum flow position. The minimum flow position of the metering valve may result in an in-flight shut-down (IFSD) of the engine. It is desirable to minimize the IFSD rate by minimizing the likelihood of a failure in the fuel control system.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop fuel control systems which result in a minimal in-flight shut-down rate for gas turbine powerplants used on aircraft.